A reverse blocking diode thyristor (RBDT) is a two terminal thyristor with shorts to the cathode-base region. The device is capable of being switched from a high impedence "off" state to a low impedence "on" state in a very short period of time. The reverse blocking diode thyristor is fully described by Chu, C. K. et al in IEEE, Industry Application Society pp. 267-82 (1973).
Reverse blocking diode thyristors are utilized in very fast switching applications such as radar modulators and laser pulse generators. The device is capable of operating at a peak current in excess of 800 amps and switching from the high-impedence, "off" state to the low-impedence, "on" state at a current turn-on rate (di/dt) of greater than 2000 amps per microsecond. The repetition rate is in excess of 1000 PPS.
Reverse blocking diode thyristors have, however, been limited by their slow turn-off time. That is, the time period before which the blocking state can be reestablished on reduction of the current below the holding current of the thyristor. The turn-off time is directly related to the lifetime of the minority carriers in the high resistivity anode-base region of the thyristor. The turn-off time of reverse blocking diode thyristors are, therefore, generally about 200 microseconds, which is too high for high frequency applications where RBDTs would otherwise be advantageous.
The present invention is an improvement on a method described and claimed in U.S. Pat. No. 3,881,963, granted May 6, 1975 to the applicants and assignee of this application. Companion patents and patent applications are U.S. Pat. Nos. 3,809,582, 3,840,887, 3,852,612, 3,872,493, 3,877,977, 3,881,964, 3,888,701, and 3,933,527 and U.S. patent application Ser. Nos. 540,208 (filed Jan. 10, 1975) now Pat. No. 3,990,091, 581,255 (filed May 27, 1975), 639,337 (filed Dec. 10, 1975) and 667,791 (filed Mar. 17, 1976), all of which are assigned to the same assignee as the present application.
In U.S. Pat. No. 3,881,963, applicants teach the radiation of thyristors with radiation sources generally and electron radiation sources preferably to decrease the turn-off time. The radiation dosage level prescribed corresponds to less than 2 .times. 10.sup.14 and preferably less than 8 .times. 10.sup.13 electrons/cm.sup.2 with 2 MeV electron radiation. More desirably, the radiation dosage level corresponds to greater than 1 .times. 10.sup.13 and preferably greater than 3 .times. 10.sup.13 electrons/cm.sup.2 with 2 MeV electron radiation. Although the irradiation may be performed with gamma and other kinds of radiation as described, it is preferred that electron radiation of any energy greater than 1 MeV be used for the radiation source.
Applicants have now found that the turn-off time of reverse blocking diode thyristors can be particularly improved by utilization of a specified narrower range of radiation dosage.